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Quantity of Heat.                              585

Reaumur divisions are adopted in Russia; those of the Centi-
grade by scientists and the Continental public \ while Fahrenheit
divisions, being used by English engineers and the English
speaking public generally, will therefore be adopted in this work,
and the Fahrenheit degree be looked upon as the unit of intensity.
Centigrade readings can be translated into Fahrenheit and vice
versfr, by the following simple formulae:

F = (C° x |) + 32     and C° = (F - 32)$,

Pyrometers are required to measure excessive temperatures,
such as those of furnaces; they are discussed on page 587.

Air thermometers are of advantage in experiments of great
delicacy, because small increase of heat will cause large expansion
of air. The instrument is usually laid horizontally, and has a
small index of coloured sulphuric acid, as at c, Fig. 601, which is
moved along the tube by the expanding air, the end B being
open to the atmosphere. The reading is considerably affected
by change of atmospheric pressure, so the barometer reading
must always be taken, and a correction made to standard
pressure. The expansion of gases is more perfect than that of
liquids. (Seepp. 587 and 1126.)

Quantity of Heat.—More or less heat motion may exist
in a body, depending on mass, heat capacity, and temperature.
The British Thermal Unit (B.T>U.) is the amount of heat required
to raise the temperature of a pound of water through one Fahrenheit
degree, the water being near its greatest density 39*1° F. This unit
represents an amount of energy equal to about 772 foot pounds.

Specific Heat.—But some bodies have greater capacity for
heat than others, that is, weight for weight, will absorb more heat
for a definite rise of temperature. Taking capacity for water as
i9 the relative capacity of another substance called its Specific
Heat^ is therefore the amount of heat in thermal units required
to raise the temperature of a pound of the substance through \)ne
degree F. Bunsen's ice calorimeter has been used to determine
various specific heats, but we shall describe the method of mixture,
which is precisely the same in principle. The body, being regu-
larly heated in a bath of steam, is removed, and put in a vessel
containing a measured weight of water at .& certain,